Olefins find widespread uses in many industries. For example, they represent the basic building blocks in such diverse uses as film and packaging, communications, construction, automotive and home appliances. These important materials are generally produced by the cracking of a hydrocarbon feedstream, which converts saturated hydrocarbons present in the feedstream into olefins.
Current cracking technologies for the production of light olefins (e.g. ethylene, propylene and, optionally, butylenes), gasoline and other cracked products such as light paraffins and naphtha can be classified into the two categories of thermal cracking (also known as steam cracking) and catalytic cracking. These technologies have been practiced for many years and are considered the workhorses for light-olefin production.
Steam or thermal cracking, a robust technology that does not utilize catalyst, produces the more valuable ethylene as the primary light olefin product. It is particularly suitable for cracking paraffinic feedstocks to a wide range of products including hydrogen, light olefins, light paraffins, and heavier liquid hydrocarbon products such as pyrolysis gasoline, steam cracked gas oil, etc. Conventional steam cracking utilizes a pyrolysis furnace, which has two main sections: a convection section and a radiant section reaction zone.
The hydrocarbon feed typically enters the convection section of the furnace as a liquid (except for light feeds which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by mixing with steam. The vaporized feed and steam mixture is then introduced into the radiant section where the cracking takes place.
Current catalytic cracking technologies employ solid acid catalysts such as zeolites to promote cracking reactions. Unlike steam cracking technology, propylene is the primary light olefin product of catalytic cracking. Accordingly, catalytic cracking would be considered as the main source for growing propylene demand.
Cracked gases typically comprise hydrogen, carbon monoxide, carbon dioxide, methane, acetylene, ethylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butanes, butenes, C5 hydrocarbons, C6-C8 hydrocarbons, non-aromatics, benzene, toluene and other heavy hydrocarbons. These gases including olefins leave the pyrolysis furnace for further downstream processing. It is necessary to separate the useful olefins, e.g., ethylene and propylene, from the rest of the product gases.
A widely practiced method for separating the product gases into various fractions involves compression, drying and fractional distillation. Typically, the product gases are compressed in multi-stage compression units wherein some heavier hydrocarbons and water are separated. The remaining product gases are then dried and chilled to separate lighter hydrocarbon fractions, e.g., ethylene and propylene, from heavier hydrocarbons. Chilling makes extensive use of ethylene and propylene refrigeration systems.
A major drawback related to the operation of the above-described olefin recovery plants relates to high-energy demands. Compression, ethylene refrigeration and propylene refrigeration are major contributors to this energy consumption.
Another method used in separating the product gases into various fractions involves the use of an absorber solvent to remove a target fraction from the remaining product gases. For example, in a demethanizer absorber, product gases containing methane typically flow upwardly within a tower in countercurrent contact with a downwardly flowing lean absorber solvent. The absorber solvent absorbs the methane from the remaining hydrocarbons in the product gases and typically exits the bottoms of the tower as (methane) rich absorber solvent. One drawback related to the use of such an absorber solvent system is that the rich absorber solvent typically contains a small, but valuable amount of desired product (e.g., ethylene) in addition to the unwanted methane. Huge expense and added operating costs can be incurred in downstream separation equipment required to separate the wanted (e.g., ethylene) products from the rich absorber solvent.